Beginner’s Guide to LTSpice - University of Toronto

1 beginner s Guide to LTSpicePages 1&2 Commands & techniques for drawing the circuitPages 3 4 Commands and methods for analysis of the circuitPage 4 Additional notes (crystals & transformers)Pages 5 9 Tutorial #1 Draw & Analyze a Transistor AmplifierPages 10 11 Tutorial #2 Draw & Analyze a Low Pass FilterPage 11 Concluding commentsDrawing putting circuit components on the drawing:(In each case, the component appears when you move the mouse. Move it to the desired locationand click. Press control-R to rotate before placing.)

2 After placing, you are ready to place anotherof the same type. Press a different key or button, or Escape to exit placing that component type.)Resistor:Press R or push the resistor :Press C or push the capacitor : Press L or push the inductor :Press G or push the ground button (triangle ground symbol)Diode:Press D or push the diode buttonOther component: Press F2 or the component button (has an AND gate on it). A menu comesup. Find your component and double-click. On the left are other sub-menus of parts you mayhave to check.

3 For example, battery is under misc .Wiring:(You can connect components by aligning their terminals when you place them on the drawing,otherwise use the wire function.)Wire:Press F3 or the wire button (pencil and blue line). Click the first point, click at anyintermediate points where you need to make 90 degree turns, click the second terminal point.( LTSpice s wire function is better than most. It doesn t want to stick to everything. But watchout for crossing intermediate terminal points. And if you intended a junction of wires and not acrossing, look for the square that indicates a junction.)

4 Assign values to components:Move the cursor over the component until the pointing finger appears. Right-click and type inthe voltage sources, just put in the basic DC value if you are doing DC analysis. For transientanalysis, click advanced, go to the left side, click Sine (usually) and enter the amplitude (peakvalue) and frequency. For AC (frequency response) analysis, go to the Small Signal AC sectionand put AC in the amplitude block and 1 in the phase assigning values, you can use p for pico, n for nano, u (letter U) for micro, k for kilo, m formilli, and MEG for mega.

5 (This isn t intended to be a complete list.) A common mistake wouldbe to say for frequency expecting MHz but getting millihertz. Use instead. Youcan also use either conventional American for a resistor or the European orinternational 4k7. You don t have to put V for volts, Hz for hertz and so on, but in most cases itwill be ignored (no error) if you componentsLTSpice labels components as R1, R2, R3, C1, C2, C3 and so on. You can change them for easeof recognition to things like Rc, Rb1, Rb2, Load and so on.

6 Right click the label and type inyour new NodesPress F4 or the label net button (a box with an A in it). Type in a name. Place the little dotover the wire or node and click. There are a couple of reasons to do can give logical names like out and in to nodes so it s easier to pick out the oneyou want to plot from a a certain node connects to many points in the circuit, you can eliminate a lot of messywiring on the drawing by giving all the nodes the same name. For example, call yourbattery (+) terminal Vcc and then put the same Vcc name on all points connecting to thatbus.

7 It has the same effect as connecting them with comments:Press T . The comment button should be selected. Type in text, ending each line withControl-M and place on drawing. Under Tools / Control Panel / Drafting Options, you can selectthe font componentsDelete:Press the delete key or F5 or push the scissors button. Move the scissors icon to thedesired component or wire or other entity and : Press move key (hand with spread fingers) or F7. Click component and move to newlocation. (It is disconnected from any wiring & components.)

8 Copy:F6 or copy button (two sheets of paper). Click, place new :Control-R. If the component is already selected (hasn t been placed), just press control-R. If it has been placed, select Move (F7), select the component, Control-R, and place :Control-E. You can rotate all day and still not get an NPN with the emitter down andthe base to the right. For this, you need themirror function.** ANALYSIS **To set-up a simulation, go to menu choice Simulation and choose Edit Simulation Command. Inevery case after you set it up and choose OK, a text command is attached to your cursor and youmust click somewhere on the drawing to make it effective for the next Run operating point for DC circuits and to check biasing and DC levels in analysis to see your waveforms in time domain, see if they are distorted, runspectrum (FFT) analysis, figure actual impedances and powers delivered and analysisto see response versus frequency for amplifiers, attenuators, filters (active orpassive) and so on.

9 Response is in dB relative to 1 volt on the op point:No parameters to set. Drop the command on the drawing and pres the Run (manrunning) button. A window with DC voltages and currents pops up. But you can see them evenmore easily by closing the box and moving the cursor over wires or nodes and reading voltagesat the bottom of the screen, or moving the cursor over devices and reading currents. Even wattsare given for resistors and analysis:For the simulation, as a minimum enter the start and stop time, maybeenough to capture 100 cycles or more (you can zoom later).

10 You also must set in your source(s)as a minimum the waveform (normally Sine), magnitude and frequency. Left-click the sourceand do this on the left side of the dialog box. Click Run, and then double click the value youwant plotted from the analysis features:From the drawing a wire or node (a voltmeter probe appears) to plot the a device (current probe appears) to plot the down the Alt key and click a device (thermometer appears) to plot the plot and drag a section of waveform tozoom the waveform name at the top of the screen to get theRMS and othercalculated a waveform name to get acursor you can drag and display values in a a waveform name and you get a drop down box that allows attaching the 1st,2nd, or both cursors.